Multiple Steam Boilers
In large hot water systems, it has become a standard to do multiple boilers to better match the ever-changing loads of the building. This idea has not caught on very well for steam systems, but the popularity is increasing. It is just as important for residential larger steam systems to do multiple boilers to achieve maximum fuel savings.
The boilers should still be sized properly according to the connected load except for one warning, each boiler should be sized for 60% of the entire load. Measure all the radiation and convert to square Feet of steam (EDR). Divide that number by two. If more than two boilers are chosen to not size the boilers for 60% of the load.
For example, 890 sq. ft X .6 = 534 sq. ft. Choose the 2 boilers from that figure.
If more than 2 boilers are used divide by the number of boilers.
Example 1280 /3=427 sq ft each.
All main steam piping must be insulated. You cannot half step into higher efficiency operation. Why install multiple boilers and have uninsulated mains making the steam prematurely convert back into condensate and must be reheated and converted back into steam.
While proper sizing the steam boiler is still the first step in maximizing fuel savings, multiple boilers are a secondary way to continue to increase comfort and reduced fuel cost.
When I say multiple boilers it means more than one boiler but in larger power burner steam boilers you can get the burners configured as low/high/low. These boilers will come on in a low fire, go to high fire and when the set point pressure is achieved it will drop back to low fir, If needed it will go back to high fire and back to low. This is all automatic. Boilers that have the ability to do low/high/low will do two boilers in one, or two boilers with low/high/low is the same as four boilers when the loads are larger or many varying size zones. The idea is when the system gets hot the steam condenses in the insulated steam mains slower and the low fire tries to match the condensation rate better.
Another choice would be full modulating boilers. These could be purchased as power burners or atmospheric gas boilers. The operation would be as above except full modulation instead of just two inputs.
Return piping to multiple boilers
Drawings are incomplete for simplicity, see manufacturers drawings
Ignore the pipe sizes and dimensional data as this drawing is for concept only. Your pipe diameters may be different. The boilers could be controlled by different means. Two most common applications would be extra pressurtrol mounted in the piping set at different pressures or staging controls which would turn boilers on and off as needed dependent on pressure. There is one control manufacturer looks at the pipe temperature on the end of the steam main.
Preferred Way - When doing multiple steam boilers, piping is also critical. We must use a dropped header and a boiler feed tank, not condensate pumps. When using pumps we do not use a Hartford Loop. Pipe into the return tee on the boiler. Each boiler must have a pump controller installed on it to start the pump and a solenoid on the return to the boiler to open and close as water is fed as the boiler requires makeup water. If two boilers are used instead of solenoid valves you may use a duplex pump. This is one tank with two pumps. One pump can be piped to each of the two boilers.
Each boiler is piped as independent boilers per manufacturer spec's and the system riser from each boiler header goes above the common header and down into the common header. This will keep any condensate from one boiler when running getting into a non-operating boiler causing the non-operating boiler to flood.
It is also advisable to add a water level steam trap(not shown) to maintain a maximum water level on the boiler that will drain into the boiler feed return tank.
Non-Preferred Way - Gravity return to each boiler, no pumps. While this is done many times without problems I really do not like to see it done. If you decide to pipe it this way I would pipe it in copper* and oversize the common return pipe by at least one pipe size. Also increase each boiler return by one pipe size also in copper. In this case you should not need the steam trap to keep the boiler from flooding. The water weight will push the water out of a non-running boiler. When we pipe this way we must pipe returns to the Hartford loop.